I think including both purchase prices would be double accounting. If you want the total cost since Hatty owned it, subtract your purchase price from his. When you sell it, add back in your purchase price minus whatever you get for it. Then you are simply including the actual depreciation in the cost of ownership. You could also account for the cost of money, but that's way beyond what I know how to do. Any CPAs here???

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1987 W201 190D

without his purchase price its around $20k